Integrated circuits are often formed on a semiconductor substrate such as a silicon wafer or other semiconductive material. In general, layers of various materials which are semiconductive, conductive, or electrically insulative are utilized to form the integrated circuits. By way of examples, the various materials may be doped, ion implanted, deposited, etched, grown, etc. using various processes. A continuing goal in semiconductive processing is to strive to reduce the size of individual circuitry components, thereby enabling smaller and denser integrated circuitry.
Memory is one type of integrated circuitry, and is used in computer systems for storing data. Such is usually fabricated in one or more arrays of individual memory cells. The memory cells might be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time, in many instances including when the computer is turned off. Volatile memory dissipates and therefore requires being refreshed/rewritten, in many instances multiple times per second. Regardless, the smallest unit in each array is termed as a memory cell and is configured to store data as one of at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store data as one of more than two selectable states.
Integrated circuitry fabrication continues to strive to produce smaller and denser integrated circuits. Accordingly, the fewer components an individual circuit device has, the smaller the construction of the finished device can be. A simple memory cell will be comprised of two conductive electrodes having a programmable material there-between. The programmable material is selected or designed to be configured in a selected one of at least two different physical memory states to enable storing of information by an individual memory cell. The reading of the cell comprises determination of which of the states the programmable material is in, and the writing of information to the cell comprises placing the programmable material in a target memory state. Some programmable materials retain a memory state in the absence of refresh, and thus may be incorporated into nonvolatile memory cells.
Some programmable materials may contain mobile charge carriers larger than electrons and holes, for example ions in some example applications. Regardless, the programmable materials may be converted from one memory state to another by moving the mobile charge carriers to alter a distribution of charge density within the programmable materials. Some example memory devices that utilize ions as mobile charge carriers are resistive RAM (RRAM) cells, which can include classes of memory cells containing multivalent oxides, and which can include memristors in some specific applications. Other example memory devices that utilize ions as charge carriers are programmable metallization cells (PMCs); which may be alternatively referred to as a conductive bridging RAM (CBRAM), nanobridge memory, or electrolyte memory.
The RRAM cells may contain programmable material sandwiched between a pair of electrodes. The programming of the RRAM cells may comprise transitioning the programmable material between first a memory state in which charge density is relatively uniformly dispersed throughout the material and a second memory state in which the charge density is concentrated in a specific region of the material (for instance, a region closer to one electrode than the other).
A PMC may similarly have programmable material sandwiched between a pair of current conductive electrodes. The PMC programmable material comprises ion conductive material, for example a suitable chalcogenide or any of various suitable oxides. A suitable voltage applied across the electrodes generates current conductive super-ionic clusters or filaments. Such result from ion transport through the ion conductive material which grows the clusters/filaments from one of the electrodes (the cathode), through the ion conductive material, and toward the other electrode (the anode). The clusters or filaments create current conductive paths between the electrodes. An opposite voltage applied across the electrodes essentially reverses the process and thus removes the current conductive paths. A PMC thus comprises a high resistance state (corresponding to the state lacking a current conductive filament or clusters between the electrodes) and a low resistance state (corresponding to the state having a current conductive filament or clusters between the electrodes), with such states being reversibly interchangeable with one another.
Another type of nonvolatile memory is known as flash memory, a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in groups. A typical flash memory array includes a large number of nonvolatile memory cells usually grouped into blocks. Each of the cells within a block may be electrically programmed by charging its programmable gate material. The charge may be removed from the programmable gate material by a block erase operation. NAND is a basic architecture of flash memory. A NAND architecture comprises at least one select gate coupled in series to a serial combination of memory cells (with the serial combination being commonly referred to as a NAND string).